trigger_error('NO_FORUM') - Fatal Error

Since updating to 3.0.6 I have noticed these errors appear in my server's error logs occasionally:

PHP Warning: strtr() [<a href='function.strtr'>function.strtr</a>]: The second argument is not an array. in /*****/forum/includes/session.php on line 2164
PHP Fatal error: template->_tpl_load_file(): File /message_body.html does not exist or is empty in /h*****/forum/includes/functions_template.php on line 63

I have not been able to determine the cause of the problems, but I was able to get a backtrace:

BACKTRACE

FILE: includes/functions.php
LINE: 4093
CALL: user->format_date()

FILE: includes/functions.php
LINE: 3630
CALL: page_header()

FILE:
LINE:
CALL: msg_handler()

FILE: posting.php
LINE: 65
CALL: trigger_error()

Line 4093 in functions.php:
'CURRENT_TIME' => sprintf($user->lang['CURRENT_TIME'], $user->format_date(time(), false, true)),

Line 3630 in functions.php is from the E_USER_NOTICE case:
page_header($msg_title, false);

Line 65 in posting.php:
trigger_error('NO_FORUM');